The regulation of the pineal gland by environmental and physiological factors is analyzed, exclusive of transmembrane and intracellular regulatory mechanisms (see Z01 HD 00095-25 LDN). The pineal gland is part of the melatonin rhythm generating system, a neural circuit which includes a circadian clock in the suprachiasmatic nucleus (SCN); the SCN is reset and entrained by light acting through the eye. An area of special current interest is proteins involved in transmembrane signal processing. The most exciting finding is that there is a daily rhythm in the abundance of mRNA encoding the alpha-1B-adrenergic receptor. This increase seems to be driven by the SCN, and is blocked by light acting through the eyes. Studies in organ culture indicate the receptor is regulated by cAMP. This second messenger is controlled by several receptors, two of which have been found to mediate increases in the abundance of mRNA encoding the alpha-1B-adrenergic receptor. This work is important because it provides an interesting mechanism through which one receptor can regulate another.